The iceberg, which separated from the northern half of the Ross Ice Shelf and has broken into four gigantic pieces, was first spied by the McMurdo Station's manager in March. Iceberg documentation usually falls under the bailiwick of the National Ice Center in Suitland, Md. The center tracks icebergs with a sophisticated array of remote sensing equipment, including satellites and radar. But with bad weather and thousands of square miles to observe, even these tools can miss a major cleft like B-15's.

To keep a closer eye on the berg, Doug MacAyeal, a glaciologist from the University of Chicago, will head to the Ross Sea to place monitoring instruments on it. In the meantime, B-15 appears to be continuing on a course toward McMurdo.

"It is an enormous piece of ice, and there is the potential it could ground in a way to block the channel," says Peter West, a communications director at the National Science Foundation who explains that moving a berg of this size "would be imponderable."

Even so, officials aren't overly concerned yet. "Nobody really knows what will happen," Mr. West says. "In that sense, there is not much to be done about it."